Photopolymerizable compositions and films containing binder, monomer, initiator and chain transfer agent are described in the prior art and sold commercially. One important application of photopolymerizable layers is in graphic arts. Photopolymerizable layers on conductive supports currently may be used as electrostatic masters for analog color proofing and are considered as promising future materials to be developed for digital color proofing applications. For the analog color proofing application, a photopolymer layer is coated on an electrically coductive substrate and contact exposed with an ultraviolet (UV) source through a half tone color separation negative. The photopolymer hardens in the areas exposed with an ultraviolet source due to polymerization and remains in a softer state elsewhere. The differences in viscosity between the exposed and unexposed areas are apparent in the transport properties, i.e., the unexposed photopolymer conducts more electrostatic charge while the UV exposed areas are substantially less conductive. By subjecting the exposed photopolymer layer to a corona discharge a latent electrostatic image is obtained consisting of electrostatic charge remaining only in the nonconducting or exposed areas of the photopolymer layer. This latent image can then be developed by application of a liquid electrostatic toner to the surface. When the developer has the opposite charge as the corona charge, the developer selectively adheres to the exposed or polymerized areas of the photopolymer layer.
Although the use of photopolymers in electrophotography has been demonstrated and many formulations can be imaged; it did not appear possible, to produce a photopolymer electrostatic master that duplicates the imaging characteristics of a printing press. The printing industry evaluates image quality and characteristics by a simple method known as dot gain curves. Dot gain means dot growth and the relationship between dot growth versus dot area in the final image is known as a dot gain curve. In a dot gain curve the dot growth is plotted along the y-axis and the actual dot size in percent along the x-axis. The perfect fidelity in reproduction corresponds to zero gain. That is, the actual dot on paper has the same diameter as the corresponding dot on the separation negative. Of course, the desired gain (standard) is the one that duplicates the printing press, a round convex curve which peaks at a gain of about 17% for 50% dots as shown in FIG. 1 appended hereto.
We have discovered that the electrical properties of a photopolymer layer can be associated with the dot gain of the final image. Low or negative dot gains curves are associated with photopolymer compositions in which the amount of corona charge retained by the imagewise exposed dot area on the master is highly non-linearly related to the percent dot area in question. These photopolymer compositions also exhibit a large difference in the conductivity between the exposed and unexposed areas. The conductivity of the exposed area is controlled by the mobility of the ions in a glassy polymer while the conductivity of the exposed area is controlled by the mobility of the ions in a softer polymer. The ratio of the mobilities between these two areas is usually about 10.sup.5 to 10.sup.6. When the conductivity ratio is too large not only is the dot gain low but the image deteriorates. This also has undesirable consequence that the image on paper does not faithfully reproduce the contact negative or the printing press.
Photopolymer compositions prior to this invention show a negative or low dot gain due to the aforementioned problems with the contrast in electrical conductivities being too large and the relationship between charge retained and corresponding percent dot area being highly non-linear. The ability of controlling dot gain permits faithfully reproducing a printing press which is a necessity for the preparation of an electrostatic proof.
It has now been found that the conductivity of both the exposed and unexposed areas can be controlled by introducing into the photopolymer composition an electron donor or an electron acceptor molecule that modifies the electrical properties of the composition and provides a dot gain curve similar to that achieved by a printing press. It has also been found that on the latent electrostatic image the charge retained by each dot is almost linearly related to the percent dot area and, as a result, the thickness of the developer layer attracted to the photopolymer master is constant, independent of the dot pattern being developed.